The present invention relates to the monitoring of tissue ablation, and more particularly, to a method and system for monitoring ablation of tissue by determining a biological response of the tissue to heat.
Cancer is a major cause of death in the modern world. Effective treatment of cancer is most readily accomplished following early detection of malignant tumors. Most techniques used to treat cancer (other than chemotherapy) are directed against a defined tumor site in an organ, such as brain, breast, ovary and colon, etc. Removal of a consolidated mass of abnormal cells is possible by surgical excision, heating, cooling, irradiative or chemical ablation.
Minimal invasive thermal therapy is a potential treatment for solid internal malignancies. This type of therapy provides for shorter hospital stays, faster recovery and better cosmetic results. In thermal therapy, heat is produced by devices inserted directly into a target site within an organ. Potentially less invasive than conventional surgery, this approach enables the treatment of tumors in otherwise inaccessible locations. Several devices have been employed for interstitial heating, including laser irradiation devices, radiofrequency ablation devices, high-focus ultrasound devices, microwave devices and the like. These devices have been shown to be capable of generating temperature elevations sufficient for thermal coagulation of tissue.
For example, radiofrequency ablation destroys tumor tissue by heat through laparoscopic application of mild, almost painless high-frequency energy applied directly to the tumor. More specifically, when an alternating electric field is created within the tissue, ions are agitated in the region neighboring the electric field source (typically an electrode). This ionic agitation creates friction and induces thermal injury to the tissue.
Radiofrequency ablation, however, is mainly applied to hepatic tumors, or tumors that are not close to a major blood vessel, due to its insufficient accuracy. The fact that the liver is a large enough organs could permit enough safety margins.
Destruction of unwanted cells via laser light can be achieved either through a direct thermal interaction between the laser beam and the tissue, or through activation of some photochemical reactions using light-activated molecules which are injected into or otherwise administered to the tissue.
The use of ultrasound for healing purposes has increased in importance. Depending on the therapy, ultrasound is applied in the form of continuous or pulsed ultrasound wave fields. The desire to generate rapid, localized temperature increases in tissue has led to the development of focused ultrasound as a method to treat tumors. In high-focus ultrasound treatment an ultrasound transducer generates focused ultrasound waves which are transmitted to the tumor. By special control of the time the focused ultrasound waves act on the tumor, resulting in an overheating of the tissue hence leading to its destruction. High-focus ultrasound can be employed by external or interstitial ultrasound transducers. To date, interstitial transducers have been developed for a variety of applications including cardiac ablation, prostate cancer ablation and gastrointestinal coagulation.
Several characteristics of the above prior art thermal therapy devices, however, limit their ability to treat large volumes or regions close to important anatomical structures. High temperatures close to the device surface often leads to undesirable physical effects of charring or vaporization in tissue. Inadequate heating can occur at the target boundary due to rapid decreases in deposited power with increasing distance from the device. Generally, the goal with interstitial thermal devices is to deliver a target-specific heating pattern which is as uniform as possible to the entire target volume of tissue, while avoiding excessive or inadequate heating.
Irrespectively of the method which is used to ablate the tumor, it is recognized that success of the treatment depends on the ability to monitor the ablation process [Hyunchul Rhim, et al., Radiographics, 2001, 21:S17-S35]. Thus, the use of minimal invasive thermal therapy is limited by the ability to monitor, hence control the destruction process precisely while it is being administered. Such precise control is required in order to minimize injury to normal adjacent parenchyma while assuring complete destruction of the offending lesion. The transfer of heat energy to the target depends on the efficiency with which the tissue absorbs the applied energy, and is therefore a function of tissue composition. Heat conduction through diffusion and perfusion processes may vary locally as a function of tissue architecture, tissue composition, local physiological parameters and the temperature itself. During ablation procedures, heat transfer characteristics may change as tissue coagulation can significantly modify heat conduction and energy absorption.
Several approaches are known in the art for monitoring the response of the treated tissue during treatment. For example, in radiofrequency ablation, commercially available devices include a thermal monitoring circuit which is integrated in the radiofrequency probe.
In another approach, impedance and capacitance-related parameters are measured and tracked during the ablation procedure to estimate tissue temperature. These techniques, however, only measure the temperature at isolated locations and cannot show the temperature distribution in the volume surrounding the destructing device. Efficient and accurate monitoring can be achieved by MRI, which can provide a reliable temperature mapping of the tissue. However, this MRI is an expensive procedure which imposes serious constraints to the surgical scenario.
In laser ablation, particularly in the area of skin disorders or in fully invasive procedures, the ablative procedure can be monitored optically using an optical fiber and a CCD camera coupled to a video monitor. A major disadvantage of this method is that it is limited to surfaces and it is difficult to apply this technique in minimal invasive procedure without significantly modifying the procedure's scenario.
An additional technique to monitor ablative procedure includes the use of ultrasound imaging. Attempts to adapt ultrasound imaging for temperature measurements include measurements of various ultrasound parameters such as the speed of sound, frequency shifts and the like. These approaches, however, have failed to provide the information required for minimizing injury to normal tissue while ablating the tumor.
There is thus a widely recognized need for a diagnostic ultrasound based monitoring method, and it would be highly advantageous to have such a method and system for monitoring ablation of tissue, devoid of the above limitations.